1. Field of the Invention
The present invention relates to an improved ion-implantation machine, the control method thereof and a process for manufacturing electronic devices.
2. Description of the Related Art
As is known, ion-implantation machines (or ion implanters) are used extensively, for example, in the semiconductor industry for implanting doping ionic species in semiconductor wafers (typically silicon wafers), with the aim of modifying the electrical characteristics of selected parts of the wafers.
Commercially available implanters comprise an implantation chamber or final station where implantation is performed in high-vacuum conditions. The implantation chamber is connected to a load lock chamber, where the wafers to be implanted are brought from an atmospheric pressure to an intermediate vacuum condition thanks to a separate pumping system, before being transferred into the final chamber. For better comprehension, FIG. 1 illustrates a known implanter 1, which includes an implantation chamber 2, a load lock chamber 3 and an intermediate valve 7 arranged therebetween.
A cryogenic pump 4 is connected, through a protective cryogenic valve, or cryovalve 5, controlled by a pneumatic-control assembly 6, to the implantation chamber 2, for creating the required high-vacuum conditions. In addition, a beamline turbopump 10 is connected to the implantation chamber 2. A pump system 9 is connected, through an isolating valve 8, to the load lock chamber 3. The implantation chamber 2 is kept in vacuum conditions both by the cryogenic pump 4, the one closest to the area where the wafers are implanted, and by another beamline turbopump provided for serving other areas, which, however, are not separate from the implantation chamber 2. As a whole, the pumping system enables, among other things, a low-pressure condition to be maintained also during species crossover, which is an intermediate step between the implantation of two different ionic species, when the implanter is kept in standby and some crossover operations are carried out, for example focusing of the ion beam on a different species to be implanted.
In addition, the implantation chamber 2 has a vent port 11, through which, during the maintenance of the implantation chamber 2, an inert gas, such as nitrogen, coming from a source 16, may be introduced into the implantation chamber 2 to enable access to the chamber for maintenance purposes, bringing it from the vacuum condition up to atmospheric pressure. An electronic control module 12 controls the pneumatic control assembly 6. In a known commercial implanter, the electronic control module 12 comprises an inlet/outlet circuit 12a and a driving circuit 12b. The driving circuit 12b is divided into a branch for driving a protective cryovalve 12b1 and an auxiliary driving branch 12b2.
As illustrated schematically in FIG. 1, the pneumatic control assembly 6, which has the task of converting the electrical signals supplied by the electronic control module 12 into pneumatic controls, comprises a first circuit 14, which is connected to the protective cryovalve driving branch 12b1 and controls the protective cryovalve 5, and a second circuit 15 connected to the auxiliary driving branch 12b2, for control of auxiliary valves, which are available for particular functions required by the user.
In implanters of this type, which are used for doping with boron and other heavier elements (e.g., arsenic), it is noted that the quality of the structures made subsequently with EPM boron implants (or threshold correction EPM) may degrade considerably. In particular, in the manufacture of flash cells including a threshold modification implant (so-called EPM implant) with boron, followed by the growth of tunnel oxide, a non-negligible degradation in the electrical characteristics of the tunnel oxide has been noted, should the EPM boron implant be carried out using implanters that also utilize arsenic among the implanted species.
On the other hand, the use of dedicated implanters for each of the different ionic species is more complex, longer and costlier so as to cause a non-indifferent increase in the cost of the final electronic devices obtained.